GABA injected into the anterior dorsal tegmentum (ADT) of the midbrain blocks stepping initiated by stimulation of the hypothalamus

Previous work showed that the activity rates of certain neurons in the anterior dorsal tegmentum (ADT) of the midbrain correlated with the onset of stepping elicited by hypothalamic stimulation. This study determined if reversible inactivation of the ADT would block locomotion elicited by hypothalamic stimulation of anesthetized rats (urethane, 800 mg/kg). GABA (concentrations 0.25–1.0 mg/μl in saline) were injected in 52 sites in 21 rats. GABA at volumes of 0.1 or 0.2 μl blocked hindlimb stepping in 18 cases. Locomotor blocks occurred within 5 min of the injection, and typically recovered within 10–20 min. The effective blocking sites were clustered around the interstitial nucleus of the medial longitudinal fasciculus. Sites more dorsal and more anterior were not as effective as sites in and ventral to this nucleus. The data are consistent with a role for the ADT of the midbrain in locomotor initiation.     

Excitatory projections from hypothalamic and midbrain defense regions to nucleus paragigantocellularis lateralis in the rat

Electrical stimulation of the hypothalamic and midbrain defense regions evoked convergent excitatory responses in neurons in nucleus paragigantocellularis lateralis (PGL) in rats anesthetized with urethane. Stimulation in PGL activated neurons in the periaqueductal gray matter (PAG) and in the hypothalamus antidromically. In the PAG the projecting neurons were localized in the defense region but in the hypothalamus they were more widely distributed lateral and dorsal to it, mainly in the lateral hypothalamic region and the zona incerta. We conclude that there are independent excitatory pathways from the midbrain and hypothalamic defense regions which relay on neurons in PGL. Stimulation in the midbrain defense region may activate the monosynaptic projection to PGL as well as polysynaptic pathways. In contrast, stimulation of the hypothalamic defense region probably excites mainly fibers of passage: the integrated pattern of autonomic changes produced by electrical stimulation may be due to activation of the efferent outflow from widely dispersed cell bodies whose axons run through the defense region.     

Afferent projections related to attack sites in the ventral midbrain tegmentum of the cat

Quiet attack was elicited by electrical stimulation of the ventrolateral midbrain tegmentum of the cat. The paw was not used other than to position or hold the rat during the bite. Bites were directed toward the head and neck region and were not accompanied by autonomic responses other than pupillary dilation and sometimes slight piloerection on the back.Horseradish peroxidase was deposited at the attack sites. Cells labeled with the peroxidase reaction product were located in gyrus proreus, gyrus genualis, nucleus accumbens, bed nucleus of the stria terminalis, bed nucleus of the anterior commissure, nucleus of the diagonal band, substantia innominata, anterior amygdaloid area, ventromedial hypothalamic area, paraventricular nucleus, perifornical hypothalamic area, lateral hypothalamic area, dorsal hypothalamic area, fields of Forel, midbrain reticular formation, superior colliculus, ventral central grey, lateral central gray, locus coeruleus, parabrachial nuclei, nucleus of the lateral lemniscus, oral pontine reticular nucleus, and the dorsal raphe. Other regions were less prominently labeled. Previous studies have shown most of these sites to have some involvement in attack.     

Projections to the caudolateral medulla from the pons,midbrain, and diencephalon in the cat

Previous research implicated neurons in the caudolateral medulla in the expression of estrous behaviors triggered by genital stimulation in the female cat. The present study identified descending pathways through which the activity of neurons in estrogen-concentrating cellular regions of the diencephalon and anterior brain stem could be transmitted to caudolateral medullary neurons. Cats received medullary injections of 50% horseradish peroxidase (HRP) in or around nucleus ambiguus. After 1 to 3 days, retrograde transport of HRP was demonstrated using tetramethyl benzidine as a chromogen. In the pons, labeled cells were most numerous in the ipsilateral parabrachial nuclei, the Kölliker-Fuse nucleus, and lateral tegmental field. In the midbrain, the central gray contained many labeled neurons bilaterally, especially at trochlear and caudal oculomotor nuclear levels. Labeled cells were also found in the midbrain reticular formation bilaterally and in the contralateral deep tectum and red nucleus. In the diencephalon, some labeled neurons were in lateral and periventricular hypothalamic regions, usually posteriorly, and many paraventricular nucleus neurons were labeled. The existence of central gray and deep tectal projections to the lateral medulla was also verified electrophysiologically by antidromic invasion. The substantial projection from the central gray to the caudolateral medulla provides a potential route for the activity of estrogen-concentrating neurons to be transmitted to cells involved in genitally triggered estrous responses because some central gray cells bind estrogen and the central gray also receives strong projections from hypothalamic estrogen-concentrating neuronal regions.     

The pathways mediating affective defense and quiet biting attack behavior from the midbrain central gray of the cat: an autoradiographic study

The purpose of this study was to describe the pathways which mediate feline affective defense and quiet biting attack behavior elicited from the midbrain central gray. In these experiments, methods of [3H]leucine and 2-deoxy-[14C]glucose (2-DG) radioautography were utilized in concert with the technique of electrical and chemical brain stimulation. Affective defense behavior elicited from the midbrain central gray is characterized by marked vocalization such as hissing and growling, pupillary dilatation, urination and piloerection. In contrast, quiet biting attack elicited from the midbrain central gray lacks overt autonomic signs observed with affective defense response as well as the stalking component which is typically associated with stimulation of the lateral hypothalamus. Nevertheless, central gray-elicited attack resulted in a directed bite of the neck of an anesthetized rat in a manner similar to that observed from the hypothalamus. Affective defense was elicited from the dorsal half of the midbrain central gray, while quiet biting attack was obtained following stimulation of the ventral half of the midbrain central gray, thus indicating a functional differentiation of the central gray with respect to these two forms of aggression. In a separate series of experiments, affective defense or quiet biting attack response was identified by electrical stimulation through a cannula electrode situated in the midbrain central gray. The affective defense responses were subsequently elicited following microinjections of D,L-homocysteic acid through the same cannula electrode in order to demonstrate that these responses were the result of direct stimulation of cell bodies within the central gray. Then, one of the following autoradiographic tracing procedures was utilized: (1) [3H]leucine was injected through a cannula electrode and the animal was sacrificed after a 4- to 14-day survival period; or (2) a 2-DG solution was systemically injected and electrical stimulation was applied through the cannula electrode in order to metabolically activate the pathways associated with each of these responses. In general, the pattern of labelled target regions as indicated by 3H-amino acid radioautography was similar to that obtained from the 2-DG autoradiographic analysis. The principal ascending pathway associated with affective defense was traced to the anteromedial hypothalamus and medial thalamus. Concerning descending projections, label was traced into the central tegmental fields of the midbrain and pons, locus coeruleus and motor and main sensory nuclei of the trigeminal complex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Midbrain cholinergic mechanisms in elicitation of hypothalamic aggressive responses in cats

1. Electrodes were implanted stereotaxically in the central gray, and the anterolateral regions of the hypothalamus, in chronically prepared cats. 2. The affective attack behaviour elicited on electrical stimulation of the hypothalamic and the midbrain regions was recorded in the caged animals. 3. Bilateral microinfusions of atropine sulphate were given in the central gray and the effect on the midbrain and the hypothalamic responses studied. 4. Cholinergic blockade by atropine sulphate raised the threshold of many of the components of attack behaviour obtained on stimulation of the central gray, and from the anterior hypothalamic regions. 5. Muscarinic receptors may be playing an important role in the central gray regions for the modulation of the aggressive responses from the hypothalamus.     

Functional mapping of the brainstem during centrally evoked bradycardia: a 2-deoxyglucose study

Autoradiographic 2-deoxy-[14C] glucose (2-DG) procedures were used to map the functional activity of the brainstem during bradycardia elicited in awake rats by stimulation of the deep mesencephalic nucleus of the midbrain reticular formation (MRF). Quantitative determinations of 2-DG uptake in 46 brainstem structures of MRF-stimulated rats were compared to those of control rats without stimulation. This paper is the first 2-DG study to map the brainstem structures involved in a heart rate response evoked by central stimulation. The structures activated in the midbrain, caudal to the stimulation site, are part of the reticular formation and the central gray. The greater focuses of labeling were concentrated on the lateral aspects of the deep mesencephalic nucleus and on the lateral divisions of the midbrain central gray. The remaining structures activated during bradycardia were all located in the caudal medulla. The largest increase was observed in the caudal nucleus ambiguus. Significant increases were also found in the dorsal motor nucleus of vagus and in the nucleus of the solitary tract. The region of the caudal inferior olive showed a small increase in 2-DG uptake, whereas structures like the raphe magnus and parvocellular reticular nucleus showed a tendency to reduce 2-DG uptake levels in the stimulated rat. It was concluded that bradycardia induced centrally by MRF stimulation may be mediated by well-defined brainstem descending pathways, direct and indirect, between the activated regions of the midbrain and the various medullary nuclei known to induce bradycardia upon electrical stimulation. The results suggest that the midbrain central gray and reticular formation may play a role as intermediates in an indirect hypothalamus-medullary circuitry for bradycardia. In addition, descending MRF information and afferent baroreceptor inputs appear to exert their inhibitory influences on heart rate via a common set of neuroanatomical substrates in the medulla.     

Locomotor sites mapped with low current stimulation in intact and kainic acid damaged hypothalamus of anesthetized rats.

To determine whether local neurons mediated the locomotor effects of electrical stimulation of the lateral hypothalamus, kainic acid injections (0.5-1.25 micrograms), intended to destroy neural somata as opposed to fibers of passage, were made unilaterally in the tuberal-posterior hypothalamus of 22 rats. The area of lesion and its contralateral homolog were mapped for locomotor stepping sites in Nembutal-anesthetized rats mounted in a stereotaxic apparatus such that locomotor stepping rotated a wheel. Stimulation (25 and 50 microA, 50 Hz, 0.5-ms cathodal pulses, 10-s trains) was delivered through 50-80 microns glass pipettes filled with 2 M saline. Contralateral to the lesion, locomotor stepping sites were common in the perifornical lateral and medial hypothalamus and less dense in the zona incerta. On the side of the kainic-acid lesion, locomotor sites were generally absent in the central part of the damaged area. If they did appear within the area of lesion, they tended to be near the border with intact tissue. In a few cases, locomotor stepping sites were found centrally located in the lesion amidst widespread loss of somata. In four rats, additional maps of anterior locomotor regions in the preoptic area ipsilateral to the lesion suggested that their descending fibers were largely spared by the kainic lesions. Local neurons appear to be major contributors to the locomotion elicited by electrical stimulation of the lateral hypothalamus, but fibers of passage may also participate.     

Hypothalamic afferent connections mediating adrenocortical responses that follow hippocampal stimulation

This study identified some neural pathways which mediate the adrenocortical responses that follow hippocampal stimulation. The increase in plasma corticosterone following dorsal hippocampus stimulation, in rats with electrodes chronically implanted under pentobarbital anesthesia, was blocked by dorsal fornix and lateral septal lesions and by small posterior hypothalamic deafferentation. Fimbria transection, lateral septal lesions, and posterior hypothalamic deafferentation, but not midbrain reticular formation lesions, also blocked the adrenocortical responses to ventral hippocampus stimulation. Our present and previous studies indicate that the dorsal and ventral hippocampal effects on the hypothalamus, which increase plasma corticosterone concentrations, are mediated by the dorsal fornix and fimbria, respectively, as well as by the lateral septum. A posterior hypothalamic input, which does not involve the medial forebrain bundle or the midbrain reticular formation is also essential for the activation of this response.     

The organization of the hypothalamic pathways mediating affective defense behavior in the cat

The purpose of this study was to describe the hypothalamic pathways which mediate affective defense in the cat utilizing the methods of [14C]2-deoxyglucose (2-DG) and [3H]leucine radioautography in concert with the technique of electrical brain stimulation. The feline affective defense response, characterized by pupillary dilatation, piloerection, ear retraction, hissing, growling and striking with the forepaws, was elicited consistently by stimulation of sites within the ventromedial hypothalamus and anterior aspect of the medial hypothalamus. In one series of experiments, 2-DG autoradiography was employed to describe the brain regions activated following stimulation of sites in the region of the ventromedial hypothalamus from which affective defense had been elicited. Ventromedial hypothalamic stimulation produced activation primarily in forebrain regions situated rostral to the level of the stimulating electrode. These structures included principally the anteromedial hypothalamus and medial preoptic area, as well as the bed nuclei of the stria terminalis and anterior commissure, diagonal band and lateral septal area. The caudal extent of activation included only the dorsal and perifornical hypothalamus at the level of the stimulation site. In a second series of experiments, affective defense sites in the anteromedial hypothalamus were stimulated and the regional distribution of 2-DG label was identified. In contrast to the results obtained from ventromedial hypothalamic stimulation, these experiments revealed a marked descending distribution of label within the posterior hypothalamus, midbrain central gray and ventral tegmental area. Results obtained from studies in which tritiated amino acids were injected into affective defense sites in both the ventromedial nucleus and anteromedial hypothalamus confirmed the general findings observed with 2-DG autoradiography. From these observations, we have concluded that the organization of the pathway mediating affective defense behavior from the ventromedial hypothalamus to the midbrain involves an initial synapse within the region of the anteromedial hypothalamus and a second synapse in the midbrain central gray substance. The significance of the anteromedial hypothalamus for the expression of affective defense behavior was considered in the Discussion.     

Increased c-Fos expression in select lateral parabrachial subnuclei following chemical versus electrical stimulation of the dorsal periaqueductal gray in rats

The parabrachial nucleus (PBN) is located in the rostral dorsolateral pons and has been identified as a critical relay for cardiovascular responses (sympathoexcitation and baroreflex attenuation) evoked by the dorsal periaqueductal gray (PAG). We examined the pattern of c-Fos protein immunoreactivity throughout the rostral-caudal extent of the PBN in four groups of anesthetized male Sprague-Dawley rats to identify the specific PBN regions activated by dorsal PAG stimulation. Both electrical stimulation and chemical (0.3 mM bicuculline methobromide) activation of the dorsal PAG elicited a selective increase in Fos-like immunoreactivity (FLI) in the superior lateral and central lateral subnuclei of the rostral lateral PBN (LPBN) relative to surgery and blood pressure control groups. In the middle LPBN chemical stimulation of the dorsal PAG selectively increased FLI in the central lateral subnucleus while electrical stimulation increased FLI in the Kolliker-Fuse area only. Finally, in the caudal LPBN only electrical stimulation of the dorsal PAG induced significant changes in FLI above control. Significant changes in FLI in the medial PBN were not observed under any experimental conditions. These results confirm neuroanatomical data demonstrating that neurons in superior lateral and central lateral subnuclei of the rostral and middle LPBN are the primary targets of the dorsal PAG. Our results also demonstrate that this descending projection to the central lateral and superior lateral subnuclei of the LPBN is in part excitatory. Finally, our results raise the possibility that neurons in the central lateral subnucleus of the middle and rostral LPBN are integrally involved in descending modulation of sympathetic drive associated with dorsal PAG activation.     

Influence of midbrain stimulation on the excitability of neurons in the medial hypothalamus of the rat

Extracellular action potentials were recorded from 1098 neurons in the medial hypothalamus of pentobarbital anesthetized male rats. Their excitability was analyzed after single 1 Hz stimulation of the midbrain periaqueductal gray (PAG) or adjacent reticular formation. Cells were also examined for their response to median eminence (ME), amygdala, lateral septum (LS) or anterior hypothalamic/preoptic area (AHA/POA) stimulation. Antidromic invasion from midbrain stimulation was recorded from 110 neurons. Eight of these neurons showed features of axon branching and displayed antidromic invasion from both midbrain and amygdala (2 cells) or AHA/POA (6 cells). Many neurons with midbrain projections displayed orthodromic responses to stimulation in the amygdala, but few responded to AHA/POA or LS stimulation. Midbrain stimulation evoked orthodromic responses from 99 medial hypothalamic neurons. Many of these cells also displayed orthodromic responses to amygdala or AHA/POA stimulation, whereas a small number were activated antidromically by stimulation in these sites. None of 42 neurons activated antidromically from median eminence stimulation were responsive to midbrain stimulation. These results provide electrophysiological evidence of reciprocal connections between medial hypothalamic and medial midbrain areas, and indicate that medial hypothalamic neurons with midbrain connections are subject to influences from other extrahypothalamic areas.     

Afferent projections to affective attack sites in cat hypothalamus

Quiet biting attack by a cat on a rat was elicited by electrical stimulation of sites in the cat's lateral hypothalamus.Horseradish peroxidase was deposited at the attack sites. Cells containing reaction products were found in gyrus proreus, anterior and central medial amygdaloid nuclei, lateral and medial preoptic areas, substantia innominata, the bed nuclei of stria terminalis, and anterior commissure. The dorsomedial area of the hypothalamus, paraventricular nucleus, suprammamillary region, and posterior hypothalamic area also contained reactive cells. In the midbrain the ventral tegmental area of Tsai, the dorsal and superior central nuclei of the raphe, central gray matter and interpeduncular nucleus were regions with reactive cells. In the pontine region, the locus coeruleus, parabrachial nuclei, nucleus of the lateral lemniscus, and the dorsal tegmental nucleus of Gudden all had reactive cells.There are many structures which send afferent projections to quiet attack sites located in the hypothalamus and the pontine tegmentum. The commonality of afferents to attack sites lends credence to the notion that a complex, distributed, interactive network underlies the neural basis of attack behavior.     

Distinct patterns of activated neurons throughout the rat midbrain periaqueductal gray induced by chemical stimulation within its subdivisions

This study provides a map of those neurons in the midbrain periaqueductal gray which are activated by chemical stimulation within different subdivisions of the periaqueductal gray. In pentobarbital anesthetized rats, the expression of the c-FOS protein was detected by immunocytochemistry and was used as a marker of neuronal activity. Microinjections of the γ-aminobutyric acid (GABA_A) receptor antagonist bicuculline (200 pmol in 50 nl) were used to increase selectively the firing rate of neurons originating from the injection site. The pattern of c-FOS immunoreactivity was highly specific for different injection sites. Dorsal injections were characterized by an extensive distribution of c-FOS immunoreactivity along the entire rostrocaudal extent of the periaqueductal gray, while ventral injections produced a much more restricted labeling. Following injection into the dorsal subdivision of the rostral periaqueductal gray, c-FOS immunoreactivity was present bilaterally in the dorsal and dorsolateral subdivisions of the rostral periaqueductal gray and was found in all subdivisions of the caudal periaqueductal gray. Dorsolateral injections at the level of the oculomotor nuclei produced strictly ipsilateral labeling in the dorsal and dorsolateral periaqueductal gray at the level of injection and throughout the ipsilateral half of the periaqueductal gray at more caudal levels. Stimulation in the ventrolateral periaqueductal gray induced FOS in the ventrolateral periaqueductal gray and the adjoining reticular formation. At rostral levels c-FOS immunoreactivity was also seen in the lateral periaqueductal gray but was absent caudal to the injection site. The identified patterns of activity in the periaqueductal gray provide a new basis for the interpretation of the diverse functional consequences of stimulation at periaqueductal gray sites. © 1995 Wiley-Liss, Inc.     

Glutamate and picrotoxin injections into the preoptic basal forebrain initiate locomotion in the anesthetized rat

This study determined the locomotor effects of glutamate and picrotoxin injections and electrical stimulation in the preoptic basal forebrain. Male rats, anesthetized with Nembutal, were held in a stereotaxic apparatus such that stepping rotated a wheel. Cathodal stimulation (0.5-ms pulses, 50-Hz frequency, 10-strain, <100 μA) was applied through a 30-gauge stainless-steel, insulated cannula to find locomotor sites. Glutamate (20 mM or 2 M) or picrotoxin (100 or 200 ng) were injected in volumes of 0.2 or 1.0 μl of saline at a rate of 1 μl/5 min. Electrical stimulation elicited locomotion (principally hindlimbs) in 32 sites which included the lateral and medial preoptic areas and the bed nucleus of the stria terminalis (BST). Stimulation in 23 sites, most in the BST and septal area, failed to produce locomotion. Stepping was elicited by glutamate and electrical stimulation in 15 sites. Glutamate was ineffective at 21 sites, at 6 of these sites electrical stimulation was effective. Longer bouts of locomotion were produced by 2 M than by 20 nM glutamate. Picrotoxin produced more intense and prolonged locomotion than glutamate. It was effective in 15 sites, at 12 of which electrical stimulation was also effective. At some ventral sites, picrotoxin-elicited stepping was continuous, at others it appeared in bursts of 5–20 s duration. At dorsal sites, the locomotor bursts were punctuated by episodes of pelvic flexion. Picrotoxin was ineffective at 12 sites, 7 of which were effective with electrical stimulation. These results indicate that activity of neurons in the preoptic basal forebrain can initiate locomotion.     

Crossed connections of the substantia nigra in the rat

The existence of crossed multisynaptic pathways that allow for the interdependent control of activity in one substantia nigra and its contralateral counterpart has been inferred from a number of recent biochemical and neurophysiological investigations. This prompted a reexamination of the connections of the substantia nigra with an emphasis on crossed inputs to and crossed projections from that nucleus. Male albino rats received 20–50-nl pressure injections of a 1% wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) solution into the substantia nigra or into surrounding areas as controls. Following a 24-hour survival period the animals were processed according to the tetramethylbenzidine protocol for the visualization of HRP. The pattern of anterograde transport of WGA-HRP after substantia nigra injections, confirming for the most part previous reports, demonstrated ipsilateral nigral efferent projections to the striatum; globus pallidus; subthalamic nucleus; the lateral dorsal, paralamellar mediodorsal, ventromedial, and parafascicular thalamic nuclei; central gray, midbrain reticular formation; superior colliculus; and peribrachial area, including the pedunculopontine nucleus. Additionally, the nigral projections to the paralamellar mediodorsal and ventromedial thalamic nuclei and to the superior colliculus were demonstrated to be bilateral. Most of these connections were confirmed by the complementary retrograde experiment. In accordance with previous reports, intranigral WGA-HRP injections retrogradely labeled neurons located in the ipsilateral prefrontal cortex, motor cortex, striatum, globus pallidus, central nucleus of the amygdala, anterior hypothalamic area, subthalamic nucleus, and dorsal raphe. Additionally, labeled perikarya were observed in the ipsilateral parafascicular thalamic nucleus, in the contralateral posterior lateral hypothalamic area, and in the ipsilateral and contralateral peribrachial-pedunculopontine area. These latter nigral afferents were confirmed with complementary WGA-HRP injections into each of the regions of origin. While bilateral peribrachial-pedunculopontine innervation of the substantia nigra has been reported in the cat there has been no previous demonstration of a crossed nigral afferent system from the contralateral posterior lateral hypothalamic area. The results are discussed with reference to the pathways that may mediate the interdependent control of the activity of neurons in the left and right substantia nigra. Additionally, the association of the substantia nigra with a variety of neuronal circuits, including the cerebellofugal, tectothalamic, thalamocortical, thalamostriatal, and basal ganglia pathways, are discussed.     

The role of the anterior hypothalamus in affective defense behavior elicited from the ventromedial hypothalamus of the cat

In the preceding paper a hypothalamic circuit subserving feline affective defense behavior was described. This circuit included an ascending component from the ventromedial nucleus to the anterior hypothalamus and a descending component from the anterior hypothalamus to the midbrain central gray substance. The present study was undertaken to test the hypothesis that the anterior hypothalamus plays a central role in the organization of this functional pathway. In the first part of this study, dual stimulation methods were utilized to demonstrate that concurrent stimulation of the ventromedial hypothalamus facilitates the occurrence of affective defense responses elicited from the anterior hypothalamus. In the second part of the study, lesions placed in the anterior hypothalamus significantly increased the latency and threshold current for affective defense responses elicited from the ventromedial hypothalamus. [14C]2-deoxyglucose autoradiography confirmed the fact that anterior hypothalamic lesions effective in blocking affective defense were placed in regions where the vast majority of ventromedial hypothalamic fibers terminate. In contrast, lesions which had little or no effect upon the latency or threshold for affective defense elicited from the ventromedial hypothalamus appeared to leave intact the connections from the ventromedial to the anterior hypothalamus. These findings are consistent with the proposed intrahypothalamic anatomical substrate subserving affective defense behavior described in the preceding paper.     

Electrical stimulation of the midbrain central gray facilitates reticulospinal activation of axial muscle EMG

EMG responses were recorded from axial muscles transversospinalis, medial longissimus, and lateral longissimus in urethane-anesthetized rats during combined electrical stimulation of the reticular formation and midbrain central gray. Central gray stimulation facilitated reticular formation-evoked EMG activity in the back muscles of the rat. Electrical stimulation of the central gray lowered the threshold for reticulospinal activation of axial muscles and could maintain firing in these muscles after the end of a reticular formation train. Units were recruited in order of size from small to large. In only one case, central gray stimulation activated axial muscles directly without reticular formation stimulation. The central gray may be important in relaying hypothalamic influences to the reticular formation, which has direct access to the axial muscles responsible for lordosis behavior.     

Predatory behavior in the cat elicited by lower brain stem and hypothalamic stimulation: a comparison

The predatory attack of a cat against a rat elicited by lateral hypothalamic, ventral midbrain and ventromedial periaqueductal gray stimulation has been compared. Regardless of the region stimulated, the attack behaviors were found to be identical with respect to response topography, preference for the rat as an attack object, the distance at which the cats would approach and attack a rat and the success in finding and attacking the rat when the cats were blindfolded. However, the minimum current required to elicit the predatory attack by periaqueductal gray stimulation was 3-4 times less than that required to elicit the same behavior by ventral midbrain and lateral hypothalamic stimulation. The difficulty in reconciling these results with the preeminent role assigned to the hypothalamus in the organization of predatory aggressive behavior was considered.     

Sex-induced fos in the medial preoptic area: projections to the midbrain

Sexual activity results in cells displaying Fos-like Immunoreactivity (FLI) in the medial preoptic area (MPOA) of male rats. This study combined retrograde tracing techniques with FLI to determine if MPOA cells displaying sex-induced FLI project to known efferent sites of the MPOA. FluoroGold was injected into the dorsal central gray, lateral central gray, ventral tegmental area, medial central tegmental field, or lateral central tegmental field of male rats that later engaged in sexual activity. Examination of FLI and FluoroGold in the MPOA revealed that the lateral region of the MPOA projected to the lateral central gray and contained smaller projections to the other regions. These findings suggest that the lateral MPOA contains secondary sex-relevant projections to the midbrain.